Lab 3.2.3 Wireless Mathematics Estimated Time: 25 minutes Number of Team Members: Students will work in teams of two or individually
Objective In this lab, the student will learn the importance of the output power of the transmitting wireless device. Students will calculate the amount of power actually transmitted from a wireless transmitting device. This will be done through the antenna element, the Effective Isotropic Radiated Power (EIRP) based on the type of antenna, cabling, connectors, and the transmitting device setting being used.
Scenario Upon completion of this lab, students will calculate potential range of the radiated wave signal transmitted by wireless devices. Students will also convert all radio frequency (RF) signal ratings into a common decibel (dB) unit in order to calculate power gain or loss.
Preparation: Prior to the lab, students should review the course materials up to 3.2.3.
Tools and Resources: 3.2.3 Interactive Activity: Calculating Decibels 3.2.3 Interactive Activity: Using Decibels
Additional Materials http://www.zytrax.com/tech/wireless/calc.htm http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00800e90fe.shtml#topi c1 http://www.cisco.com/en/US/products/hw/wireless/ps4570/products_installation_guide_chapter09186 a0080184b5a.html http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.ht ml http://www.access.gpo.gov/nara/cfr/waisidx_02/47cfr15_02.html Students should research the Cisco website for the following information if needed:
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Technical specifications of the power output in decibels (milliwatts) of the wireless devices used. Access points (APs)and client adapters are examples of these devices.
•
Technical specifications of the gain in decibels referenced to an isotropic antenna (dBi) of various wireless device antennas.
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Technical specifications of the gain/loss in decibels (dB) of various wireless device cables
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Technical specifications of the gain/loss in decibels (dB) of various wireless device connectors. These connectors are necessary when cables have to be joined for longer cable lengths.
Fundamentals of Wireless LANs - Lab 3.2.3
Copyright 2003, Cisco Systems, Inc.
Maximum Power Levels ETSI Band (GHz)
2.4
5.15 – 5.25
5.25 – 5.35
5.470 – 5.725
5.725 – 5.825
EIRP
100 mW
200 mW
200 mW
1000 mW
25 mW
20 dBm
22 dBm
22 dBm
30 dBm
14 dBm
Band (GHz)
2.4
5.15 – 5.25
5.25 – 5.35
5.470 – 5.725
5.725 – 5.825
Conducted Power
-
50 mW
250 mW
N/A
1000 mW
EIRP
4000 mW
200 mW
1000 mW
P2MP – 4 W ( 36 dBm )
36 dBm
22 dBm
30 dBm
P2P – 200 W ( 53 dBm )
FCC
Step 1 Calculate the decibel rating. The decibel (dB) measures the power of a signal as a function of its ratio to another standardized value. The symbol is often combined with other symbols to represent what values are being compared. For example: dBm where the decibel value is being compared to 1 milliWatt, and dBw where the decibel value is being compared to 1 Watt. For example: Power (in dB) = 10 * log10 (Signal/Reference) Where: Signal is the power of the signal (for example 50 mW) Reference is the reference power (for example 1 mW) In the example: Power (in dB) = 10 * log10 (50/1) = 10 * log10 (50) = 10 * 1,7 = 17 dBm Since decibels are ratios comparing two power levels, simple math can be used to manipulate them for designing and building networks. Using the previous example: Power (in dB) = 10 * log10 (5 * 10) = (10 * log10 (5)) + (10 * log10(10)) = 7 + 10 = 17 dBm
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Fundamentals of Wireless LANs - Lab 3.2.3
Copyright 2003, Cisco Systems, Inc.
Complete the missing values below. If help is needed, use the “Calculating Decibels” Interactive Activity. An increase of:
A decrease of:
3dB
Produces: Double transmit power
3dB 10dB
Half transmit power 10 times the transmit power
10dB _____
Decreases transmit power 10 times 32 times the transmit power
____ 20dB
Decreases transmit power 32 times _________ times the transmit power
20dB ____
Decreases transmit power ________ times 316 times the transmit power
____ 30dB
Decreases transmit power 316 times _________ times the transmit power
30dB
Decreases transmit power ________ times
A decrease of:
Produces:
Additional Practice: An increase of: 5dB
_________ times the transmit power 5dB
40dB
_________ times the transmit power 40dB
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Decreases transmit power ________ times
Fundamentals of Wireless LANs - Lab 3.2.3
Decreases transmit power ________ times
Copyright 2003, Cisco Systems, Inc.
Step 2 Calculate the delivered power.
Another way to look at this formula is where Pfinal = Pref * 10 (dB/10). In the example above, a 2.4 GHz AP is set to 100mW and has a 2.2 dBi antenna. Now calculate the following scenarios. Use the “Using Decibels” Interactive Activity AP Power
Antenna
Power output (in mW)
1 mW
2.2 dBi
_____________
5 mW
6 dBi
_____________
50 mW
9 dBi
_____________
100 mW
6 dBi
_____________
100 mW
22 dBi
_____________
1. What is the maximum allowable output power in dBm and Watts for the 2.4 GHz band? FCC
_____________________________________________________________________________ ETSI
_____________________________________________________________________________ ___________ (Other Regulatory domain)
_____________________________________________________________________________ 2. What is the maximum allowable output power in dBm and Watts for the 5 GHz band? FCC
_____________________________________________________________________________ ETSI
_____________________________________________________________________________ ___________ (Other Regulatory domain)
_____________________________________________________________________________ 3. Why is it necessary for regulatory bodies to define maximum power levels?
_____________________________________________________________________________ 4-6
Fundamentals of Wireless LANs - Lab 3.2.3
Copyright 2003, Cisco Systems, Inc.
4. What power levels can be set for the 2.4 GHz radio on an AP 1100? 350? 1200?
_____________________________________________________________________________ 5. What power levels can be set for the 2.4 GHz radio on an PCM 350 NICs?
_____________________________________________________________________________ 6. What power levels can be set for the 5 GHz radio on an AP 1200?
_____________________________________________________________________________ 7. What power levels can be set for the 5 GHz radio on the BR 1400?
_____________________________________________________________________________ 8. What are the approximate dBm values for each of the following power levels? dBm
mw
___dBm
1mW
___dBm
5mW
___dBm
20mW
___dBm
30mW
___dBm
50mW
___dBm
100mW
Step 3 Calculate the total power output of the wireless device The radiated (transmitted) power is rated in either dBm or Watts. Power coming off an antenna is measured as Effective Isotropic Radiated Power (EIRP). EIRP is the value that regulatory agencies such as the FCC or European Telecommunications Standards Institute (ETSI) use to determine and measure power limits in applications such as 2.4 GHz wireless equipment. EIRP is calculated by adding the transmitter power (in dBm) to antenna gain (in dBi) and subtracting any cable losses (in dB.) The dB notation can also be used to describe the power level rating of antennas: dBi for use with isotropic antennas (theoretical antennas that send the same power density in all directions) and dBd when referring to dipole antennas. Antennas are compared to this ideal measurement, and all FCC calculations use this measurement (dBi.) Dipole antennas are more real world antennas. While some antennas are rated in dBd, the majority use dBi. The power rating difference between dBd and dBi is approximately 2.2; that is, 0dBd = 2.2dBi. Therefore, an antenna rated at 3dBd is rated by the FCC (and Cisco) as 5.2dBi. Example 1: Description
Cisco Part Number
Power
AP
AIR-AP1200-A-K9
20 dBm
Antenna gain:
AIR-ANT2012
6 dBi
Antenna Cable loss:
AIR-CAB050LL-R
-3.35 dBi
20dBm + 6dB – 3.35dBi = 34dBm
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Fundamentals of Wireless LANs - Lab 3.2.3
Copyright 2003, Cisco Systems, Inc.
EIRP = 22.65 dBm Example 2: Description
Cisco Part Number
Power
A Cisco Aironet Bridge
AIR-BR350-A-K9
20 dBm
50 foot antenna cable
AIR-CAB050LL-R
3.35 dB loss
solid dish antenna
AIR-ANT3338
21 dBi gain EIRP
37.65 dBm
9. Which example is permissible according to local regulatory guidelines?
_____________________________________________________________________________
Calculate the EIRP for the following:
AP Output 20-dBm 17-dBm 15-dBm 13-dBm 7-dBm
Antenna Gain 12 dBi 5.2 dBi 21 dBi 8.5 dBi 2.2 dBi
0 dBm
2.2 dBi
EIRP
10. What are the primary hardware factors involved that affect signal distance?
_____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________
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Fundamentals of Wireless LANs - Lab 3.2.3
Copyright 2003, Cisco Systems, Inc.
